Q. We are facing the problem of wet adhesion test failures on painted parts. Can you tell me the root cause of these failures? P.G.
A. You didn’t tell me the nature of the substrate and the type of paints you are using. Furthermore, you didn’t tell me exactly what you were testing, adhesion to a pretreated substrate or intercoat adhesion. So I’ll answer both cases. During the wet adhesion test, soaking causes water to be absorbed by the paint film and expand it. This expansion can cause the film to separate from the substrate. This happens in both intercoat adhesion and adhesion to a pretreated substrate.
In the case of intercoat adhesion, the two paint films may expand at different rates. If the composition of these films is not the same, they could have different rates of absorption and expansion. If they are the same, the water may not have been absorbed by the underlying film to the same extent, and the degree of expansion may be different. Again these differences will cause the films to separate and lose adhesion. In the case of adhesion loss to a pretreated substrate, the paint film can follow the same pattern by expanding and separating from the substrate. However, remember that the greatest single cause of adhesion loss to a pretreated substrate is improper surface preparation. This rule applies to both of the aforementioned cases and could be the real cause of the wet adhesion test failure.